Seismic Morphology of a Late Paleozoic Isolated Carbonate Platform: Karachaganak Field, Republic of Kazakhstan

Abstract

A new PSDM seismic volume improved the image of internal geometries in the Devonian to Permian Karachaganak isolated platform. Seismic geometries reveal platforms that aggrade, coalesce during progradation, and backstep. These observations are supplemented by core descriptions, FMI bed dips, MDT pressure data, and wireline log interpretation, and define a stratigraphic framework that guides correlations from the seismic to well-scale.

Late Devonian to early Mississippian seismic reflectors onlap a Mid-Devonian faulted structural high and exhibit dipping slope reflectors that down-lap away from the same high. Late Devonian to Early Mississippian aggradation results in a broad (26 km x 9 km), steep-sided platform that was exposed. In the Middle Mississippian, six distinct circular cores of chaotic seismic character are surrounded by concentric, dipping reflectors in depth slice. In cross-section the cores are characterized by mounded to chaotic reflectors and are 1-2km wide and 300m thick. These cores are interpreted to be small, isolated buildups initiated during a subsequent sea level rise. The concentric, dipping reflectors are interpreted as clinoform reflectors representing progradation as the buildups amalgamated. By the end of the Mississippian, the smaller, isolated buildups had coalesced into two larger scale flat-topped platforms (Western and Main Buildups) separated by an embayment. The embayment was largely filled prior to a long period of exposure (24 My).

Permian depth slices exhibit nine distinct, circular cores of chaotic seismic character, along with areas of dipping reflectors, which make up an area 2-10 km across. In cross-section cores are mounded to chaotic reflectors where mounds down lap onto the top Carboniferous or on shallower reflectors connected to other mounds within the Permian (100-1000m thick, 1km - 4km wide). The Permian structures are interpreted to have back-stepped and initiated as a series of smaller aggradational mounds, then partially amalgamated into larger buildups (100-1000m thick, 2km – 10km wide) through progradation of clinoforms and growth of down-stepped mounds on inherited slopes.

Karachaganak’s multi-stage history of platform growth generates complex reservoir architecture. A new, enhanced seismic image integrated with a wide range of existing data has promoted improved mapping and enabled a better realization of connectivity and predicted facies trends for reservoir modeling.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018